Make image Cache eviction strategy less aggressive in iced_wgpu

Instead of trimming unconditionally at the end of
a frame, we now trim the cache only when there is a
cache miss.

This way, images that are not visible but still a
part of the layout will stay cached. Eviction will
only happen when the images are not a part of the
UI for two consectuive frames.

wgpu/src/image/atlas/allocator.rs

@@ -33,6 +33,10 @@ impl Allocator {
     pub fn is_empty(&self) -> bool {
         self.allocations == 0
     }
+
+    pub fn allocations(&self) -> usize {
+        self.allocations
+    }
 }
 
 pub struct Region {

wgpu/src/image/atlas/layer.rs

@@ -11,4 +11,12 @@ impl Layer {
     pub fn is_empty(&self) -> bool {
         matches!(self, Layer::Empty)
     }
+
+    pub fn allocations(&self) -> usize {
+        match self {
+            Layer::Empty => 0,
+            Layer::Busy(allocator) => allocator.allocations(),
+            Layer::Full => 1,
+        }
+    }
 }